Extend validity checks for references and Box

[celinval]

• A reference or Box must be aligned, it cannot be dangling, and it must point to a valid value (in case of dynamically sized types, using the actual dynamic type of the pointee as determined by the metadata). Note that the last point (about pointing to a valid value) remains a subject of some debate.

• The metadata of a wide reference, Box, or raw pointer must match the type of the unsized tail:

  • dyn Trait metadata must be a pointer to a compiler-generated vtable for Trait. (For raw pointers, this requirement remains a subject of some debate.)
  • Slice ([T]) metadata must be a valid usize. Furthermore, for wide references and Box, slice metadata is invalid if it makes the total size of the pointed-to value bigger than isize::MAX.

https://doc.rust-lang.org/reference/behavior-considered-undefined.html